Current Stem Cell Research & Therapy - Volume 2, Issue 4, 2007

MicroRNAs (miRNAs) are a newly discovered, yet powerful mechanism for regulating protein expression via mRNA translational inhibition. Loss of all miRNA function within mice leads to embryonic lethality with a loss of the stem cell population in the epiblast and failure to form a primitive streak. These data suggest that miRNAs play a major role in embryonic development. As critical regulation of protein expression is also important for controlling the balance between self-renewal and differentiation in stem cells, the study of miRNAs within this model system is rapidly expanding. New data suggest that stem cells have discrete miRNA expression profiles, which may account for, or contribute to, the intrinsic stem cell properties of self-renewal and pluripotency. Specifically, miRNAs have been implicated in downregulation of cell cycle checkpoint proteins during germ stem cell division. Other data demonstrate that changes in miRNA expression can promote or inhibit stem or progenitor cell differentiation within different cell lineages, including hematopoietic cells, cardiomyocytes, myoblasts, and neural cells. In this review we detail the established functional roles of miRNAs in the embryonic and adult stem cell model systems. Finally, we explore new techniques that exploit endogenous miRNA processing and function for applications in basic and clinical research.

Regenerative medicine opens new avenues and promises towards more effective therapies for autoimmune disorders. Current therapeutic strategies for type I diabetes focus on three major directions, with distinct advantages and disadvantages: arrest of autoimmunity, islet transplantation and generation of neoislets. There is mounting evidence that candidate stem cells residing in the hematopoietic compartments participate in regeneration of pancreatic islets following chemical and autoimmune injury in vivo. The apparent major mechanisms include immunomodulation, revascularization, support of endogenous β-cell regeneration and differentiation into insulin-producing cells. Review of the current evidence suggests that some divergent observations depend primarily on the experimental design, which both limits and accentuates developmental events. The flood of publications reporting negative results appears to reflect primarily suboptimal experimental conditions for differentiation of putative stem cells, rather than limited developmental plasticity. Stem cells modulate the course of autoimmune diabetes through multiple mechanisms, including de novo generation of units capable to sense, produce and secrete insulin. Therefore, the charged debate over controversies surrounding developmental plasticity should not impede attempts to design curative therapies for this disease.

Although the use of stem cells in cell-replacement therapies by transplantation is obvious, another equally important and interesting application of stem cells is to use them in disease modeling. Disease models serve as a platform to dissect the biochemical mechanisms of normal phenotypes and the processes which go awry during disease conditions. Particularly in complex, multigenic diseases, molecular studies lead to a greater understanding of the disease, and perhaps more targeted approaches for therapies. Stem cells provide an ideal in vitro system in which to study events related to development at the molecular and cellular level. Neural stem cells have been used as excellent models to study the mechanisms of differentiation of cells of the central nervous system. These studies may be particularly relevant to diseases of complex etiology such as psychiatric illnesses, neurodegenerative diseases and brain tumors. Stem cell-derived systems are also being developed to create models of cardiovascular disease. The application of stem cells to the study of cardiovascular illnesses, and vertebrate heart development, is discussed.

Although myoblast transplantation in patients with ischemic heart failure results in a significant improvement of cardiac function, subsequent studies have consistently shown the myotubes formation in the absence of electromechanical coupling with the neighboring host myocardium, accompanied with the short-term release of paracrine effectors from implanted cells. One major pitfall of using myoblasts is that transplanted cells do not differentiate into cardiomyocytes, which may cause the inherent proarrhythmogenic events. Therefore, whether a discrete subpopulation in heterogeneous muscle-cell cultures is responsible for substantial cardiovascular regeneration has yet to be investigated. We describe here the isolation of progenitor cells from human skeletal muscle. These cells proliferated as non-adherent myospheres in suspension and displayed early embryonic factors and mesenchymal cell-like characteristics. Flow cytometric analyses demonstrated that CD56/N-CAM/Leu-19, a neural cell adhesion molecule abundantly present in myoblasts, was absent in myospheres but was expressed in an adherent cell population containing myogenic precursors. Myospherederived progenitor cells (MDPCs) differentiated in culture to produce cardiac, smooth muscle, and endothelial cells. Transplantation of MDPCs into ischemic hearts in NOD/scid mice promoted angiogenesis with substantial cardiovascular regeneration. Our results provide a foundation to further study the cell and biological function of human MDPCs which may have potential therapeutic implications.

It is estimated that almost 1 in 3 individuals in the United States might benefit from regenerative medicine therapy. Unfortunately, embryonic stem (ES) cell therapies are currently limited by ethical, political, biological and regulatory hurdles. Thus, for the foreseeable future, the march of regenerative medicine to the clinic will depend upon the development of non-ES cell therapies. Current sources of non-ES cells easily available in large numbers can be found in the bone marrow, adipose tissue and umbilical cord blood. Each of these types of stem cells has already begun to be utilized to treat a variety of diseases. This review will show that cord blood (CB) contains multiple populations of ES-like and other pluripotential stem cells, capable of giving rise to hematopoietic, epithelial, endothelial, and neural tissues both in vitro and in vivo. Cumulatively, the identification and isolation of these populations of pluripotent stem cells within cord blood represents a scientific breakthrough that could potentially impact every field of medicine, via their use in regenerative medicine. Thus, CB stem cells are amenable to treatment of a wide variety of diseases including cardiovascular, hepatic, ophthalmic, orthopaedic, neurological and endocrine diseases.

Mesenchymal Stromal Cells (MSC) are key candidates for cellular therapies. Although most therapeutic applications have focused on adult bone marrow derived MSC, increasing evidence suggests that MSC are present within a wide range of tissues. Umbilical cord blood (CB) has been proven to be a valuable source of hematopoietic stem cells, but its therapeutic potential extends beyond the hematopoietic component suggesting regenerative potential in solid organs as well. There is evidence that other stem or progenitor populations, such as MSC, exist in CB which might be responsible for these effects. Many different stem and progenitor cell populations have been postulated with potential ranging from embryonic like to lineage-committed progenitor cells. Based on the confusing data, this review focuses on a human CB derived, plastic adherent fibroblastoid population expressing similar characteristics to bone marrow derived MSC. It concentrates especially on concepts of isolation and expansion, comparing the phenotype with bone marrow derived MSC, describing the differentiation capacity and finally in the last the therapeutic potential with regard to regenerative medicine, stromal support, immune modulation and gene therapy.

Since the first successful cord blood transplant was performed in 1988 there has been a gradual increase in the use of cord blood for hemopoietic stem cell transplantation. Worldwide, over 8,000 unrelated cord blood transplants have been performed with the majority being for children with hemopoietic malignancies. Transplantation for adults has increased but is limited by the low number of nucleated cells and CD34+ cells within a single cord blood collection. Cord blood hemopoietic stem cells are more primitive than their adult counterparts and have high proliferative potential. Cord blood ex vivo expansion is designed to improve transplant outcomes by increasing the number of hemopoietic stem cells with long term repopulating potential and their differentiated progeny. However, despite a large amount of research activity during the last decade, this aim has not been realized. Herein we discuss the rationale for this approach; culture methods for ex vivo expansion, ways to assess the functional capacity of ex vivo generated hemopoietic stem cells and clinical outcomes following transplantation with ex vivo expanded cord blood.